The flu is a contagious respiratory infection caused by influenza viruses that circulate worldwide, producing roughly one billion cases each year. It kills between 290,000 and 650,000 people annually, making it one of the most significant infectious diseases on the planet. Unlike the common cold, the flu hits fast and hard, often knocking people out for a week or more.
How the Flu Differs From a Cold
Both the flu and the common cold are respiratory illnesses, but they’re caused by different viruses and feel quite different. Flu symptoms come on abruptly. You might feel fine in the morning and be flat on your back by dinner. A cold, by contrast, creeps in gradually over a day or two and tends to stay milder throughout.
The classic flu symptoms include fever or chills, cough, sore throat, runny or stuffy nose, muscle and body aches, headaches, and fatigue. What sets the flu apart is the intensity: the body aches can be severe, the fatigue can be crushing, and fevers often run higher than with a cold. Colds rarely cause the deep muscle pain and exhaustion that define a bad flu.
Types of Influenza Virus
Four types of influenza virus exist: A, B, C, and D. Only A and B matter for most people. These two types drive the seasonal flu epidemics that sweep through communities every winter. Influenza A is the more dangerous of the pair because it’s the only type capable of causing pandemics, the large-scale global outbreaks that have periodically reshaped public health history.
Influenza C causes mild illness and doesn’t trigger epidemics. Influenza D primarily infects cattle and isn’t known to make people sick at all.
How the Virus Infects Your Body
Flu viruses target the cells lining your airways, from your nose down through your bronchial tubes. The virus latches onto receptors on the surface of these cells using a protein on its outer shell. Once attached, the cell absorbs the virus, pulling it inside. The virus then hijacks the cell’s machinery, forcing it to produce copies of the virus instead of doing its normal job. Those copies burst out and infect neighboring cells, spreading the infection deeper into the respiratory tract.
This process triggers your immune system’s inflammatory response, which is what produces most of the symptoms you feel. The fever, aches, and fatigue aren’t caused directly by the virus destroying tissue. They’re largely your body’s own defense system going to war.
How the Flu Spreads
Most adults with the flu shed virus from their upper respiratory tract and are contagious from the day before symptoms appear through roughly five to seven days after symptoms start. That one-day head start before you feel sick is part of what makes flu so effective at spreading. You can pass it to others before you even know you’re infected.
The virus travels through respiratory droplets produced by coughing, sneezing, or talking. You can also pick it up by touching a contaminated surface and then touching your face.
Who Faces the Greatest Risk
For most healthy adults, the flu is miserable but not dangerous. For certain groups, though, it can become life-threatening. The people at highest risk for severe complications include:
- Adults 65 and older
- Children younger than 5, with the highest hospitalization and death rates among infants under 6 months
- Pregnant women, including up to two weeks after the end of pregnancy
- People with chronic conditions such as asthma, COPD, heart disease, diabetes, kidney or liver disorders, and sickle cell disease
- People with weakened immune systems from conditions like HIV or from treatments like chemotherapy
- People with a BMI of 40 or higher
- People with neurological conditions or disabilities that affect muscle function, lung function, or the ability to cough and clear the airways
- Residents of nursing homes and long-term care facilities
Racial and ethnic disparities also play a role. Black, Hispanic or Latino, and American Indian or Alaska Native populations face higher rates of flu-related hospitalization, driven by differences in healthcare access, underlying health conditions, and other systemic factors.
Complications Worth Knowing About
The flu’s most dangerous complication is pneumonia, and it comes in two forms. The virus itself can cause viral pneumonia, but the more common and often deadlier scenario is a secondary bacterial infection that takes hold after the flu has weakened your airways and immune defenses. The bacteria most frequently responsible are the same ones that cause other common respiratory infections, ear infections, and in severe cases, bloodstream infections and meningitis.
MRSA-related pneumonia following the flu is particularly concerning, carrying a mortality rate around 30%. Even less severe secondary infections, like ear infections or sinus infections, are common enough that a sudden worsening of symptoms after you’ve started to improve should prompt attention. If you feel like you’re getting better and then spike a new fever or develop worsening chest symptoms, that pattern often signals a bacterial infection on top of the original flu.
Antiviral Treatment
Several prescription antiviral medications can shorten the flu and reduce its severity, but timing matters enormously. They work best when started within 48 hours of your first symptoms. After that window, the benefit drops significantly for otherwise healthy people, though hospitalized patients and those with severe illness can still benefit from later treatment.
The most widely used option is an oral medication taken twice daily for five days. Other options include an inhaled version, a single-dose oral tablet, and an intravenous form used in hospitals. All of them work against both influenza A and B. These medications don’t cure the flu instantly, but they can shave roughly a day off your illness and, more importantly, reduce the risk of serious complications in high-risk patients.
For most healthy adults who are past the 48-hour window and managing their symptoms at home, antiviral treatment typically isn’t necessary. Rest, fluids, and over-the-counter medications for fever and pain remain the foundation of care.
Flu Vaccine Effectiveness
The flu vaccine is reformulated every year to match the strains expected to circulate, which means its effectiveness varies from season to season. During the 2025–26 season, the vaccine reduced flu-related doctor visits by 38–41% in children and adolescents and by 22–34% in adults. Protection against hospitalization was 41% for children and around 30% for adults.
Those numbers might seem modest, but they represent a meaningful reduction in severe outcomes across an enormous population. A 30% reduction in hospitalizations, applied to tens of millions of infections, prevents hundreds of thousands of hospital stays each year. The vaccine’s protection tends to be strongest in children and weaker in adults over 65, whose immune systems mount a less robust response. Even when the vaccine doesn’t prevent infection entirely, it often reduces the severity of illness.
Vaccination remains the single most effective preventive measure against the flu. Basic hygiene, including frequent handwashing and avoiding close contact with sick individuals, adds another layer of protection, especially during peak flu season from December through February in the Northern Hemisphere.